This invention relates to recovery of oil from a subterranean reservoir through the use of surfactant flooding.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the initial oil still in the formation. This has led to the use of what is commonly referred to as secondary recovery or water flooding wherein a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir because of the inability of the water to wet the oil and the capillary retention of the oil. Accordingly, it has been suggested to use a surfactant in the water flooding processes. It has been found that the use of surfactants can reduce the interfacial tension between the oil and the water to such an extent that substantially increased quantities of oil can be displaced.
However, there are other variables involved in addition to the wetting ability of the water, and in fact conventional surfactant flooding techniques also leave substantial amounts of oil in place.
Further efforts to better remove residual oil from subterranean deposits have focused on the use of microemulsions. In accordance with this technique, a microemulsion is prepared by mixing oil with brine and surface-active agents. Some systems are capable of achieving good results in removing oil from the pores of a subterranean formation. However, there is an obvious drawback to any system for recovering oil which involves the injection of oil which has already been recovered back into the ground. Another drawback is high surfactant usage due to adsorption of the surfactant on formation rock.